I took the plunge and attempted to build some myself. This was my first attempt at SMD soldering using solder paste. It went much better than I had expected. I highly recommend using the stencil and solder paste method. The stencil adds some extra cost, but is well worth the investment. Electrotrain’s assembly notes on the github page give a good overview. The stencil is only needed for the side with the FPGA. For the other side applying the solder paste from the syringe or using a toothpick by hand will work well. I would recommend having a solder wick/braid on hand (with a very fine braid) even if using the stencil as 2 of my 3 boards had bridged pins on the FPGA. I found having a magnifying glass and eye loupe necessary to inspect the solder paste and finished solder joints. I would also spend some time watching the eevblog youtube videos on smd/smt reflow soldering before getting started.
I found that using the stencil was pretty forgiving. If you mess up applying the solder paste you can wipe it off and start over. When placing the parts on the solder paste some fine adjustment could be done to get it aligned correctly before pressing it down into the paste to hold it in place.
The PCBs, stencil, and parts to build 3 boards cost about 200 USD. All of the parts were available from Mouser.com (quick and reasonable shipping in the US at least). The PCB was easily ordered from Oshpark with the eagle .brd file. I ordered a mylar stencil from Oshstencil, again all that was needed was the eagle .brd file.
I tested operation of the oven several times with junk boards and some ebay smd breakout boards to get a feel for the operation. For the cost of a new toaster oven and the solder paste you might be able to join a local hackerspace/makerspace for access to a proper reflow oven. If they have a laser cutter you could even make the stencil there instead of ordering.
PCB in jig to hold in place
Stencil aligned over pcb and taped in place
FPGA side with paste
Close up of FPGA with paste
Bridged pins after reflow, used the cut end of solder braid to wick out the excess with soldering iron.
Closeup of FPGA and clock oscillator. Clock does not have exposed pins so need to use oven or hot air.
Front with paste applied by hand
Front with paste and parts
Front reflowed with hot air gun.
(I modified the PCB eagle file to add the pull up resistor on the SD card. In the process I deleted the power trace to the SD card. Fixed with the black jumper wire.)