Each level of the satellites interior is separated by 4 standoffs. These can either be male/female screw type or simple Alu. female unthreaded type. The Cubesat document DS_CSK_Midplane_Standoff_711-00331-B.pdf recommends using "M3 threaded rods and simple spacers inside a CubeSat Kit" due to the lower mass. Below are the required parts for both scenarios using Mouser/Harwin part numbers. (Pumpkin says they use McMaster for spacers but they do not carry them in 15mm length.
Check pdf for more info: http://www.keyelco.com/pdfs/M55p67.pdf
Cubesat Kit supplied part dimensions: 14.9mm(L) x 4.4mm(OD) x 3.3mm(ID)
Male/Female Threaded Type:
15mm FM430 to Radio: 4 x R30-3001502 4 x 534-24291 (Stainless) In stock with mouser
25mm Radio to Antenna: 4 x R30-3002502 4 x 534-24294 (Stainless) In stock with mouser
15mm Antenna to Payload: 4 x R30-3001502 4 x 534-24291 (Stainless) In stock with mouser
15mm Payload to Battery: 4 x R30-3001502 4 x 534-24291 (Stainless) In stock with mouser
12mm Battery to Chassis: 4 x 534-24290 (Stainless) In stock with mouser
Totals:
4 x 534-24290 (Stainless) In stock with mouser
12 x 534-24291 (Stainless) In stock with mouser
4 x 534-24294 (Stainless) In stock with mouser
Rods and Spacers:
FM430 to Radio (15mm): 4 x R30-6201514 (alu) or R30-6011502 (brass)
Radio to Antenna (25mm): 4 x R30-6202514 (alu) or R30-5002502 (brass)
Antenna to Payload (15mm): 4 x R30-6201514 (alu) or R30-6011502 (brass)
Payload to Battery (15mm): 4 x R30-6201514 (alu) or R30-6011502 (brass)
