Prism99

Have you tried blocking it with starch? Sharon Schamber had some videos on how to do this, but I can't find them. Here are the steps I would take with a panel.

Mark the outlines of what the panel *should* be on the matte side of a piece of freezer paper using a permanent Sharpie (so the lines don't bleed). This would be a square or rectangle -- whatever the cut size is that you are looking for.

Hopefully you have an ironing board with enough padding that you can stick pins in it. If not, you will have to add a layer of padding to it (batting would work fine, especially something stable like Warm & Natural). Iron the freezer paper to the top of your ironing board (or the top of the padding).

Heavily spray starch the panel (sopping wet).

Pin the wet panel to conform to the marked shape on your ironing board, stretching as needed. You can use lots of pins. If you can't get it exactly right, get it as close as you can without huge ripples in it. Stretch as necessary.

At this point I would let it dry naturally. You can place a fan on it so it dries faster.

Once it's dry, remove enough pins so you can maneuver your iron around on it. Use a pressing cloth and steam to gently reheat the starch in an area and press it down. Once the entire block has been done, let it rest until cool.

Basically what this does is allow you to manipulate the panel into correct shape (or as near to correct as you can get it). The starch holds the fabric in place while you work on the quilt.

In order to keep this shape, you need to do some quilting. The quilting stitches secure the panel to the batting, which is much stronger, and the batting will keep the panel straight. This means you need to do some quilting in the panel. If you quilt only around the outer edges of the panel, the panel might balloon up out of shape when washed. (You could still apply more quilting at that point, but it would be a hassle.)

The above sounds like more work than it actually is. You may not be able to get the panel exactly squared, so you still may need to do the framing as described above. It will be less wonky to frame, however.

Edit: You can try pressing while the panel is wet. If you do, be sure to use a pressing cloth so you don't accidentally scorch the starch. Although scorched starch will wash out later, it's hard to tell the difference between scorched starch and scorched fabric.