Generate random number sequence with certain entropy

I need to generate partly random sequence of numbers such that the sequence overall has certain entropy level.

E.g. if I would feed the generated data into gzip it would be able compress it. And in fact, this would be the exact application for the code, testing data compressors.

I'm programming this in C++ and first idea that came into my mind would be to initialize bunch of std::mt19937 PRNGs with random seed and choose one randomly and make random lenght pattern with it. The std::mt19937 is reset each time with same seed so that it always generates same pattern:

#include <iostream>
#include <random>
#include <vector>

int main() {

    std::random_device rd;
    std::vector<std::mt19937> rngs;
    std::vector<int> seeds;

    std::uniform_int_distribution<int> patternrg(0,31);
    std::uniform_int_distribution<int> lenghtrg(1,64);
    std::uniform_int_distribution<int> valuerg(0,255);

    for(int i = 0; i < 32; ++i) {
        seeds.push_back(rd());
        rngs.emplace_back(seeds.back());
    }

    for(;;) {
        // Choose generator and pattern lenght randomly.
        auto gen = patternrg(rd);
        auto len = lenghtrg(rd);
        rngs[gen].seed(seeds[gen]);
        for(int i = 0; i < len; ++i) {
            std::cout << valuerg( rngs[gen] )<<"\n";
        }
    }
}

Above code mets the first requirement of generating compressable randomness but the second is harder: how to control the level entropy/randomness?