Who engineeers this stuff????

check this page
http://www.jpcycles.com/search/search?N=0&Ntt=fork caps&Ntk=All
there are ones from Drag Specialties and Colony that look like solids of your hollow design. hope this helps.

The only fork tube plug they have listed was this one

http://www.jpcycles.com/product/231-149

they only have one picture of it, can not tell from the angle of it if it is solid. Sent them an email to find out....

Thanks