I was able to take the legs off the extension table that came with my 6600 and use it with an Arrow table. But it sounds like you might be wanting the newer table anyway, and there's nothing wrong with that. I'd probably get the new one.

425.00 is a lot of money to spend on something you have to kludge together.

It is good for you to think down the road but I don't know why you would want to upgrade your DSM in 5 years. I am still quilting on my Bernina 1530 and as long as it continues to run fine (which I hope and pray every night), then why buy a new machine?? I am using the sewing desk that my Dad bought me from my HS graduation in 1970 that came with a Singer. I had a pressed wood cut to be a table top for the Bernina in the 90s so she would fit. Unfortunately, I bought a plastic insert and after the years, it finally cracked. I now use the metal table top insert accessory and the Bernina still fit fine in the desk. After all these years of quilting, I don't need a thread cutter or a stitch regulator or a laser beam. HAPPY quilting everyone !!

In 2013 I bought a new machine that was larger and heavier than what I had. The table I had been using bowed from the weight of the machine. I purchased an Arrow cabinet for the larger machine along with the acrylic insert. I believe I would think about the future and buy the larger cabinet. It may be a bit more but might actually save money down the road.

My Brother PQ 1500 sits in Horn cabinet not quite made for that type of machine. Too small to really get at side bobbin. No way to get a flatbed adapter for it. (My Pfaff has the adapter, fits both Horn and Koala tables)

I like those ready to assemble storage units sold by some craft stores. Some drawers come with divider strips that I don't need. I keep them for shims and just found that I have enough to put in the gap behind the machine. They have enamel paint and don't catch like foam can plus they are exactly the right size.
I haven't adjusted the lift for the 1500. It's still set for the Pfaff. Once I'm sure I won't be switching the Pfaff between cabinets, I'lll probably adjust the lift. You can adjust those lifts. Look on Koala and Horn's sites and you should find instructions.

Steady Betty has this for another problem gap:
http://www.steadybetty.com/products/betty-bridge

This is the table I have for my 6600, and I LOVE it: http://www.sewperfecttables.com/sewing-table-6500.shtml

It's made specifically to fit this machine, and it's solid as a rock. They are not kidding when they say it's an industrial table. It doesn't vibrate or shake AT ALL, which is something that always bothered me with previous tables I tried.

It's $499 + $50 shipping so it's a little bit more than what you're looking at, BUT it's custom made for your machine so no inserts needed. It's pretty easy to assemble, too - I put mine together all by myself, while I was home recovering from hip replacement surgery. :)

Jo C gets the award for the smartest idea of the day! I have a table with the open space in the back of the machine. Whenever I'm piecing, the fabric goes right down the hole. I am definitely going to be doing that.

I think the gap will cause problems and a lot of frustration even when regular piecing let alone fmq. If you would be happy with "creating something, as suggested," then get the table. But I don't think that sounds like much of a discount for something you will have to make work. When we upgraded to a Bernina 880 we also purchased the largest Koala cabinet made for it. I don't regret the extra cost and love my table. But I anticipate this machine meeting my needs for the rest of my life at age 64. And I don't fmq but the large table is wonderful when putting on binding or borders on large quilts.

I had the same problem with a cabinet I inherited from my Mom. No way was I giving that up. I put a couple of 2 x 4s in the gap in the back. These were left over from some project in the basement.

This makes sense to me. If you don't have that, it's inexpensive to by Styrofoam or just cover it with a piece of vinyl andtape it down.