What size T-slot nutsbolts do I want to get for the 20x20 Aluminum

M8 X 25MM T-BOLT STAINLESS M8 X 25MM T-BOLT STAINLESS STEEL has a slot in the thread to

T-slot bolts DIN 787 go back · Products · Operating parts

m8 t slot bolts bolt visually indicates when it is properly positioned in the T-slot Material Steel Thread M8 Version Galvanised Length 16 Profile series 30 40

m88 slot T-BOLT M8 X 25 T-BOLT