The torah forbids creating artifacts used in the beis hamikdash (based on the verse lo taasun iti (Exodus 20:19), see Talmud Avoda Zara 43a, Rambam Beit Habechira 7:10 and Shulchan Aruch YD 141:8). This would seem to include the trumpets, which the gemara elsewhere learns could be made of any material. Therefore, can one create, buy, sell or own these instruments?
The Minchas Chinuch in Mitzva 254 writes (my own translation):
The trumpets could very well be included in the latter category of utensils that do not have a specific form. Rambam (Kli Hamikdash 3:5) writes that they must be made from silver and ideally out of a single block but places no constraints on the measurements.