If Adapteva can point at a few good/compatible heatsink candidates that would be v. helpful. Sourcing/shipping from your local shop would tend to be cheaper.
I think that the ATS maxiFLOW heatsinks look to have good heat profiles and come in range of sizes.
So for example ( see digi-key US site) :
ATS1130-ND (15mm x 15mm x 17.5mm) for Epiphany
ATS1131-ND (17mm x 17mm x 17.5mm) for Zynq/ARM
A bit tall perhaps for the case, but lower profiles are available.
I hesitate to recommend them 'cos they seem to cost a bomb!!! - esp with shipping and VAT
- but they DO look rather cool.
- and I mean Cool
tery