This is my favorite article on the topic:
these two in particular:
“Athletes can also perform heat acclimation training prior to traveling to a hot environment. An example of this occurred before the 1994 Track Cycling World Championships, which were held in Palermo, Italy. The US National Team trained on indoor trainers in a hyperbaric chamber at the Olympic Training Center in Colorado Springs, CO, which was set to replicate the high heat, high humidity, and low altitude (sea level) of Palermo. The strategy worked, and the team earned two golds, two silvers, and a bronze on the way to finishing third in the medal count.”
“Simpler methods of acclimatization training include wearing additional layers of clothing, sitting passively in hot sauna (heat exposure) or training in a hot room.”
A little more detailed info in this multi-part series
however it doesn’t offer any specific advice regarding indoor training environment, outside what I quoted above.
My personal experience is I’ve seen more benefits from increasing the temperature of the room to at least 26-28C, while still using a fan. That might not be so easy where you live. There are studies that have been mentioned on the TR podcast about using a sauna, instead of training in a warm room.
Hope that helps, and have fun in Spain!