How much space do you have to work with (L x W x H)? How are your fabrication skills? Glass or acrylic? Cutting glass and/or acrylic is not for the light of heart or patience; you have to take your time on cuts and be a perfectionist. You can do it, you just have to have a good idea of what you want, like how much macro algae, how big is your skimmer, heaters, pumps, anoxic zones, filters, dosing pumps, etc. etc. Seriously try and think of everything that will be going into the sump and refugium. The better idea of what you have in total will give you an idea of how big you need it to be. I've been thinking of converting my 20 long into a sump/refugium, as it has a greater surface area to work with (30" x 12" x 13") than my old Eshopps sump so I'll list a couple of links for suppliers for this stuff that I've found, if you're inspired to take this project on. I won't lie, it's probably going to be cheaper to just buy a refugium sump as opposed to building one, after you consider the time in labor, waiting for shipments, frustration, extra material, mistakes, etc. If you're looking for something custom, but don't want to take the time that's necessary, check out Fluid Designer's.
Acrylic Supplier, good pricing, but don't forget about shipping!
ePlastics.com
This is just an online retailer, but it will give you a comparison price to work off of:
GlassCages.com
A good DIY on how to build acrylic aquariums from Chiclid Forums:
Building an Acrylic Aquarium
Fluid Designers website (note that the site is under construction, but give them a call anyway and see what they would ask for building a custom refugium like you want.)
Fluid Designers
Another good link for a DIY refugium/sump
sump/refugium design
