Allegiant Air does not have a published musical instruments policy. However, it does appear that they allow the purchase of an additional seat to secure your larger instrument if you prefer to bring it on-board the aircraft. This is mentioned on the Allegiant Air website, as follows:
[AMZ asins='B0CMR5YLMN,B01BGX82G8,B083TJQ1XN,B000T07XFG']
[AMZ asins='B0CMR5YLMN,B01BGX82G8,B083TJQ1XN,B000T07XFG']
Passengers wishing to secure a musical item, a child restraint device for an infant, etc., must purchase an additional ticket.