My version of making custom sized frame for painting on thin paper that needs to be pressed between glass and back-plate. Construction is made strong enough for very heavy 6mm organic glass.

What you need for the task:

Wooden laths of 2 sizes (thinner for back and thicker for front, make sore front lathe is wide enough to hold picture in place)
Small hacksaw
Angle template for sawing
Electric drill
Small screws
Glass
Wooden glue (optional)
Pressed cardboard or any solid sheet as a back-plate
Metal corners for furniture
Metal wire

Step 1: Measure Your Painting and Get Appropriate Glass and Backplate

In my case I got solid plastic sheet to put as back-plate, but any solid material could be used.

I used 6mm glass instead of 2mm (non-reflective), because for such large frame thinner glass is much more fragile and also 6mm is musch cheaper.

Next measure and buy wooden lathes for dimensions of painting

Step 2: Make a Back Frame

Cut 4 straight pieces of more narrow wooden lathe and bind them using metal corners.

Important!

Drill first holes, then put screws, because such small wooden lathes are very fragile and could split in two in the middle.

I advise also to drill through the lathe and choose screws that goes out a little bit through other end (1.5-2 mm) to bind later with front part of frame. If you use such method do not tighten screws first. Also Make sure you try out putting frame around painting for adjustments before drilling and screwing all corners.

Step 3: Make Top Frame

Cut wider wooden lathes using 45 degrees template for cutting. Make sure you cut pieces with reserve (longer than needed) - it is always possible to make them shorter, but not longer. Then try to put them on front side.

Next apply some glue and screw them from other side.

Important!

Make sure you use screws that are not too long so that they do not stick out on front end

Drill holes for screws first. Do not drill too deep in front lathe.