If you belong to a quilt guild/group chances are someone has somethign you can use free, even if you have to go to their home. sometimes others want to get rid of their bulky light boxes as they take up so much room to store. HINT: one of the quilt groups I bleong to has a very large light box donated for members' use ... so the owner didn't have to store it anymore.
You can always go the old fashioned and really cheap way and use painters tape and a window with good sunlight coming in.
For the clear portion a number of idea: the acryllic extention table, buy some clear plastic from a local supplier, use the glass from a picture frame [can find cheap at thrift stores,] buy a small piece of glass from a glass store as they may have smaller pieces available from other projects, likewise buy glass from a framing store, use a glass top from a pation table or from an end table [if you have one with such a topper], do you have access to a glass storm window? ... just let your imagination go wild.
Some quilters use a large almost clear plastic bin [Joanns or thrift sttore/garage sale], cut a groove for a light cord on one end, put their light facing up on a flat surface, and then put the plastic bin upside down over the lamp.
For holding up the glass: I use soup cans with my Ott-like lamp under the clear acrylic I have, open table leaves, between folding snack tables [if use acryllic], and more.
I expect you'll get a lot of different answers here.
ali