Carriage of cargo in pilot compartments is prohibited except that in aircraft having a maximum take-off weight not more than 5700 kg, cargo may be carried in an unoccupied control seat.
PlankBlender, in your situation 20.16.2.6.1 implies that you can only carry cargo in the cabin of aircraft less than 5700kg and if so it
may be carryed on an unoccupied seat.
The Archer is less than 5700kg so you
may carry it in the cabin and it
may be carryed on an unoccupied seat. You do not have to carry it on a seat. With the chair removed the cargo must be secured as to 20.16.2.4.1. Cargo may obstruct an emergency exit if suitable other exits are available, 20.16.2.4.2. There is a table in part 105 somewhere that tells you how many exits are required for number of occupants.