Making a sauna?

I have built an infrared sauna with 3, 250 watt heat lamps. The enclosure is 3.5'x5'x6.5'. With the lamps I find it is taking too long to heat up and it doesn't get hot enough, at least I didn't really sweat after half an hour. I have a few questions if someone could help. Does anyone have any suggestions on building some type of enclosure like a tent or box. I have a tarp that I can use to cover it? How big can this be with the lamps I have? How hot should the enclosure in this type of sauna get for good therapeutic value?

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