The pre-veneered MDF here can only be got in either 13mm thick or 19mm thickness. Would you say 13mm MDF is thick enough, or should I go 19mm thick in case mouth resonances are gonna be a problem for this 300hz horn? (29cm long, something like 1088cm^2 mouth area I think).

The horn in the picture uses 16mm plain MDF, cant get the veneered MDF in 16mm though just 13mm and 19mm.

I dont like putting on veneer, just building the horns again from pre-veneered MDF is the easiest for me.