Here is a nice clear picture of the basic joint used in constructing things from their website.
There is a washer on both sides.
Their bill of materials calls for stainless bolts, stainless washers and stainless self-locking nuts:
B O L T S & N U T S
M6 SELFLOCKING NUTS, stainless (DIN985) - ca. 150 pieces (TWOSEATER: 200 pieces)
M8 SELFLOCKING NUTS, stainless (DIN985) - ca. 5 pieces (TWOSEATER: identical)
M8 NUTS NORMAL, stainless - ca. 5 pieces (TWOSEATER: identical)
M6 and M8 BOLTS with Hexagon head, stainless, (DIN931):
6mm x 140 mm - ca. 2 pieces (TWOSEATER: 3 pieces)
6mm x 90 mm - ca. 15 pieces (TWOSEATER: 20 pieces)
6mm x 90 mm - ca. 5 pieces in DIN 933 (full thread) (TWOSEATER: identical)
6mm x 60 mm - ca. 60 pieces (TWOSEATER: 120 pieces)
6mm x 65 mm - ca. 10 pieces (TWOSEATER: identical)
6mm x 40 mm - ca. 25 pieces (TWOSEATER: identical)
8mm x 120mm, 2 pieces (TWOSEATER: identical)
8mm x 100mm, 2 pieces (TWOSEATER: identical)
M6 WASHERS - ca. 300 pieces (TWOSEATER: 400 pieces)
M8 WASHERS - ca. 10 pieces (TWOSEATER: identical)
Concerning galvanic corrosion:
Stainless steel fasteners in aluminium plates or sheets are normally considered safe, whereas aluminium rivets or bolts holding stainless steel parts together is an unwise combination, as there is a practical risk of corrosion.
An example of the safe use of stainless steel and aluminium together is where stainless steel fasteners and hold down bolts are used to secure aluminium roadway or bridge parapet guards.
Even with no insulation between the metals, there should be little risk of corrosion.
A drill press will make life much better for a project like this.