I just bought a model SP-25 (I think) 5X7 Elwood Enlarger in pretty good shape b ut it is missing a part(s). In the parts list (it came with a parts list brochu re!) its called a "light filter and heat absorbing glass". This fits in the bott om of the light housing over the negative stage and I think it diffuses the ligh t. Without this glass in place the light is very uneven. Is there some special p attern to this glass? Could I have one made from heat resistant frosted glass an d would that work? Is this enlarger a worthwhile project or should I not waste m y time? It seems pretty sturdy and was cheap. I would appreciate any tips from a nyone with experience with these machines.