I have about a hundred different picture frames, this is part of my collection. I have kept the sizes to the standard A5, A6 and A7. I even have some A4 sizes. 1.5mm Perspex can be bought online very cheaply, even cut to size. I bought A4 sheets and cut them to size using a Perspex cutter. I have also included a back plate which holds any picture firm with the printed retaining lugs on the frame. A fun print none the less, and you can come up with some pretty nice results by using PLA silks etc.
You will notice that in most of my designs I have a large flat surface. This surface is the print surface, layer 1. This large print surface area helps to maintain bed adhesion, reduce warpage and keep the print still. In my experience this is the best way to go with regards to reliable printing, I like to aim for a 99% hit rate on my prints but in reality I probably achieve about 95%.
For example the other night I was printing a large piece for my electric wheel chair project. We were at about the 2 and a half day mark and unusually, the temperature dropped quite severely during the night and to cut a long story short when I check the work that morning the PLA 300 x 300 x 400 piece about half way done had warped quite badly at the base. These types of things are very difficult to counter in the home environment. My point is that even though I had a large print surface in contact with the printer bed my object still warped! I was unlucky that time. In general the larger the print surface in contact with the bed, the more reliable the print.