I believe stainless steel fasteners would be fine with aluminum in
this situation.

More info below quoted from http://www.bssa.org.uk/topics.php?article=89


"Aluminium and stainless steel together also appears to be a bi-
metallic corrosion risk, from the 'nobility' table.
With this combination the affect of relative surface area on corrosion
is important.

A large area of 'cathode' relative to 'anode' will accelerate the
anodic corrosion. Although aluminium is anodic to stainless steel,
large relative surface areas of aluminium to stainless steel can be
acceptable, dependant on local conditions.
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.

In contrast, in a marine environment, severe localised pitting
corrosion to the aluminium treads has been observed where un-insulated
stainless steel bolts were used to secure the treads in place.
On the same ladder however, bolts with sound insulating washers did
not show any pitting on the surrounding aluminium."