You could use solid metal, screw-in lens caps, or caps for/from filter stacks.
http://www.ebay.com/itm/52mm-Metal-Screw-in-lens-Cap-Filter-Stack-Strorage-Case-Nikon-Lenses/320956773093?_trksid=p2047675.m1982&_trkparms=aid% 3D333005%26algo%3DRIC.FIT%26ao%3D1%26asc%3D15538%2 6meid%3D8288963965580181830%26pid%3D100009%26prg%3 D7473%26rk%3D4%26sd%3D220989334514%2
Find a cap with diameter and thread size that fits your shutter, then drill whatever size hole
in the cap adequate for the Holga lens.
You could basically epoxy the Holga lens to the metal screw-on filter cap.