If you're going to improve the design, you have to start by analyzing what is wrong with the current design. Make a list of the things you think are wrong and then a list of possible fixes. You can post those lists here and ask for feedback. I'm sure you'll get plenty.
For starters, I seriously doubt that a plastic frame is going to make a crappy printer better. It can't possibly be THAT crappy.
If you want to see an example of a printer built to reliably deliver quality prints, check the link in my sig, below. It may help point out some of your printer's faults and may give you some ideas for design improvements. My printer still has plenty of faults- it's big, heavy, noisy, not very fast, and a little expensive, but it consistently delivers very high quality prints.
Ultra MegaMax Dominator 3D printer: [
drmrehorst.blogspot.com]